1. Field of the Invention
This invention relates to a glass composition which looks like or can be regarded as crystal glass although it does not contain lead (Pb) and is suitable for use to produce tableware or like articles.
2. Description of the Related Art
Conventionally, lead-containing crystal glass is used popularly as crystal glass for use to produce tableware or like articles. The glass contains a large amount of lead, and the characteristics of it, in short, such characteristics that "it is massive", that "it has a high light refraction index and exhibits a high dispersion value of light" and that "it melts readily", are provided principally by PbO. While the content of PbO varies widely, it generally ranges from 24 to 26 percent by weight.
However, it has come into question recently that lead, which is particularly injurious to the human bodies, undergoes dissolution from lead-containing crystal glass. For example, in California, U.S.A., a complaint was filed asserting that the public organization "has not given a clear warning against exposure to lead" arising from lead crystal glass used to produce tableware, decanters and like articles. According to the FDA (Food and Drug Administration) standards of the U.S.A., the regulated value for the dissolution amount of lead has been revised to a severe value.
Meanwhile, in Japan, the dissolution amount of lead from lead-containing crystal glass is regulated in the "Official Announcement No. 84 of the Ministry of Health and Welfare Based on the Food Hygiene Law", and lead-containing crystal glassware makers manufacture lead-containing crystal glass so that the dissolution amount of lead may not exceed the regulated value even where the content of PbO in the lead-containing crystal glass is within 24 to 26 percent. However, it is forecast that the regulated value may be restricted to a progressively severe value in Japan.
Therefore, in order to suppress dissolution or lead from lead-containing crystal glass, Japanese Patent Laid-Open Application No. Heisei 3-205325 has proposed a technique wherein the surface of a piece of lead-containing crystal glass tableware is covered with a protective film which contains silicon oxide as a principal component. However, since the technique requires secondary working after working of glass, a rise of the production cost cannot be avoided.
Another technique has been proposed and is disclosed in Japanese Patent Laid-Open Application No. Heisel 2-141434 wherein the surface of a crystal glass vessel is covered with a lead-free glass layer. With the technique, however, since two kinds of glass are melted simultaneously and sprayed to the crystal glass vessel to mold the lead-free glass layer, there is a problem in that the molding is low in efficiency and besides presswork cannot be employed and so forth.
A further technique is disclosed in Japanese Patent Laid-Open Application No. Heisei 4-50133 wherein the amount of an alkali metal or metals to be added to the total amount of PbO, an alkali earth metal or metals and ZnO is limited in order to reduce dissolution of lead. However, the reduction reached is only about one half.